Cali. Biol. Mar. (2013) 54 : 649-655 Antarctic urchin Ctenocidaris speciosa spines: lessons from the deep Ana I. CATARINO1, Virginie GUIBOURT1, Claire MOUREAUX1, Chantai DE RIDDER1, Philippe COMPERE2 and Philippe DUBOIS1 H) Laboratoire de Biologie Marine, Université Libre de Bruxelles, CP 160/10; av FD Roosevelt 50; 1050 Brussels, Belgium. Telephone: +32-2-650.29.70, fax: +32-2-650.27.96, E-mail:[email protected] O) Département des Sciences et Gestion de IEnvironnement, Université de Liège, Bât. B6c, allée du 6 Août 10; 4000 Liège, Belgium Abstract: Ocean acidification is leading to changes in the oceanic carbonate system. As a result, calcium carbonate saturation horizon is shallowing, especially at high latitudes. Biogenic high magnesium-calcites could be particularly vulnerable, since their solubility is either similar or greater than that of aragonite. Cidaroid urchins have magnesium-calcite spines covered by a polycrystalline cortex which becomes exposed to seawater when mature (not covered by an epidermis). However, deep species live at low calcium carbonate saturation states, especially at high latitudes. We describe here the morphology and the magnesium content of Ctenocidaris speciosa spines collected at different depths from the Weddell Sea (Antarctica) and relate the features with seawater calcium carbonate saturation. We observed that the spines cortex of C. speciosa presented a thicker inner cortex layer and a lower [Mg2+] below the aragonite saturation horizon. We suggest that the cortex of cidaroid spines is able to resist to low calcium carbonate saturation state. Résumé : Les épines de l ’oursin antarctique Ctenocidaris speciosa : leçons venues des profondeurs. L’acidification des océans apporte des changements dans le système des carbonates de l’eau de mer. En conséquence, l’horizon de saturation du carbonate de calcium est moins profond, en particulier aux hautes latitudes. Les calcites magnésiennes biogènes pourraient être particulièrement vulnérables car leur solubilité est soit similaire soit supérieure à celle de l’aragonite. Les oursins cidaroïdes ont des piquants formés de calcite magnésienne et sont couverts par un cortex polycristallin qui se trou­ ve exposé à l’eau de mer quand les piquants sont matures (non recouverts par un épiderme). Cependant, les espèces profondes vivent à de faibles états de saturation du carbonate de calcium, en particulier aux hautes latitudes. Nous présentons ici la morphologie et la concentration en magnésium des piquants de Ctenocidaris speciosa prélevés à différentes profondeurs dans la mer de Weddell (Antarctique), en relation avec les caractéristiques de saturation en carbonate de calcium de l’eau de mer. Nous avons observé que le cortex interne des piquants de C. speciosa est plus épais et montre une concentration plus faible en magnésium en-dessous de l’horizon de saturation de l’aragonite. Nous émettons l’hypothèse que le cortex des piquants des cidaroïdes est capable de résister aux faibles états de saturation du carbonate de calcium. Keywords: Ocean Acidification • Antarctica • Ctenocidaris speciosa • Cidaroid spines • Calcium carbonate saturation state 650 ANTARCTIC CIDAROID SPINES Introduction urchins as their growth, calcification rate and survival can be either impaired or not (Shirayama & Thornton, 2005; Anthropogenic carbon dioxide (CO 2 ) emissions to the Ries et al., 2009). atmosphere are being taken up by the oceans, modifying the Sea urchin spines are essential structures that play an seawater carbonate equilibrium and resulting in reduced pH important role in locomotion, defense, food gathering and and carbonate ion concentration, decreasing in turn the inter and intra-specific interactions (Hétérier et al., 2008; saturation state of calcium carbonate minerals (Orr et al., Moureaux et al., 2010). Furthermore, they possess the 2005). This phenomenon, referred to as ocean acidification, ability to regenerate once broken or removed (Moureaux et implies a decrease of 0.3-0.4 pH units expected by the end al., 2010). The cidaroid primary spines have particular of the 21st century if atmospheric C02 emissions reach characteristics when compared with those of other sea concentrations around 800 ppmv (Caldeira & Wickett, urchins (“euechinoids”). First, they are characterized by the 2005). Subsequently, the surface waters may become presence of a monocrystalline stereom surrounded by a undersaturated towards the mineral forms more soluble polycrystalline cortex (Märkel et al., 1971). Moreover, an than calcite, such as aragonite and magnesium calcite (Mg- epithelium covers the shaft only until the cortex has been calcite), whose solubility increases with Mg2+ content deposited and when the spine becomes mature the (Andersson et al., 2008; Zeebe & Wolf-Gladrow, 2001), epithelium disappears (Prouho, 1885; Märkel & Röser, within the 21st century, especially at the poles (Caldeira & 1983), leaving the cortex exposed to environmental Wickett, 2005; Orr et al., 2005; Andersson et al., 2008). physical and chemical conditions. Subsequently, the shaft Several authors have studied the link between the becomes heavily colonized by epibionts making these sea magnesium to calcium ratio (Mg/Ca) in biogenic urchins islands of biodiversity, especially in deep sea where magnesium calcites and the pH or carbonate concentrations muddy substrate predominates just as in the Southern of seawater. In studies where C 02 was bubbled and TA Ocean (> 60° S) (Hétérier et al., 2008). As most models of ocean-carbon cycle predict that the shallowing of the (total alkalinity) remained constant no relationship in CaC03 saturation horizons due to increased anthropogenic foraminifera magnesium content was observed neither with the carbonate ion saturation (A[C032_], i.e. the difference C02 emissions will be more important at higher latitudes between [C032-] measured in situ and [C032-] saturation) (Anderson et al., 2008), the effects of acidification on the cidaroid spines will be relevant to the ecology of this nor with [C032-] (Dissard et al., 2010). Using field region. foraminifera specimens, Rathmann & Kuhnert (2008) did In this study we described the morphology and the not see a clear influence of [C032-] on skeleton Mg/Ca, magnesium content (along depth) of cidaroid spines from while Yu & Elderfield (2008) showed that Mg/Ca increased field specimens collected in the Weddell Sea (Antarctica), with carbonate ion saturation A[C032_], and that it lowered complementing works such as the ones from of Märkel et below a threshold value in one of the two studied species al. (1971) and David et al. (2009). We also evaluated these (but a dissolution effect was discarded). Furthermore, in a differences in an ocean acidification context, according to study where 14 species, including sea urchins, crustaceans, calcite and aragonite saturation states of seawater as mollusks, corals, among others, were reared at different proxies for Mg-calcite. calcite and aragonite saturation states, the Mg/Ca ratio of the newly formed shells and skeletons did not differ between control values and low saturation states except for Material and Methods a coralline red algae where it decreased and for a serpulid worm tube where it increased (Ries, 2011), but with no link to calcification rate observed. Field samples and data Sea urchins and other echinoderms have a well- Eleven primary aboral and ten oral spines from the species developed high-magnesium calcite endoskeleton whose Ctenocidaris speciosa (Mortensen, 1910) were studied. precursor is a transient amorphous calcium carbonate They were provided by the Paris Natural History Museum (ACC) phase, a CaC03 form 30 times more soluble than (France) and Laboratory of Marine Biology (Brussels calcite (Politi et al., 2004). In fact, they are considered to be University, Belgium) (Table 1). The specimens were particularly vulnerable to ocean acidification effects trawled in the Weddell Sea (Antarctica) during EPOS 3 (in (Kurihara, 2008). Interestingly, many sea urchin species 1989) and ANT XV/3 (in 1998) campaigns (research ship reach depths under the saturation horizon of aragonite, and R/V Polarsten, Alfred Wegener Institute for Polar and most likely of high Mg-calcite (David et al., 2005; Orr et Marine Research, Germany). The sampling depths were al., 2005). Diverse responses to low pH and calcium 237, 602 and 810 m for EPOS 3 and 1286-1681 m for ANT carbonate saturation states have been observed in sea XV/3 (Table 1). All specimens (1 individual from 237 m, 2 A.I. CATARINO, V. GUIBOURT, C. MOUREAUX, C. DE RIDDER, P. COMPERE, P. DUBOIS 651 Table 1. Ctenocidaris speciosa. Origin of sampled specimens (Weddell Sea, Antarctica) from the Paris Natural History Museum (MNHN), France, and the Brussels University (ULB), Belgium. Expedition Date of Depth Latitude Longitude Collection Museum No. aboral No. oral Collection (m) reference spines spines EPOS 3 18 January 1989 237 60°37.6’S 46°58.1’W MNHN E cE s9310 3 2 EPOS 3 3 February 1989 602 74°39.9’S 29°31.3’W MNHN E cE s9308 2 3 EPOS 3 4 February 1989 810 75°32.4’S 29°53.0’W MNHN E cE s9235 3 2 ANT XV/3 4 February 1998 1286-1681 73°28.05’S- 22°30.0’W- ULB 3 3 73°28.04’S 22°40.05’W for 810 m and 3 for 602 m) from EPOS 3 were stored dried, aqueous 1 pm diamond suspension (1PS-1MIC, ESCIL, while those (3 individuals) coming from the ANT XV/3 France). The polished slices were carbon-coated (rotary campaign were stored in ethanol. Further museological evaporator Balzers MED-010, Liechtenstein). Dispersive samples were not possible since the applied study methods X-ray analysis spectra were obtained in an environmental were destructive. SEM (XL30 ESEM-FEG-FEI Philips, The Netherlands) Total dissolved inorganic carbon (DIC) and total operating at lOkV and at a working distance of 10 mm. The alkanity (TA) data from the sampling sites were obtained analyses were repeated three times for each region on each using the data base from the WOCE Southern Ocean Atlas spine. They were performed on mineral zones of about 1-10 (Orsi & Whitworth, 2004).